B737NG flight crew reported descending below cleared altitude on arrival into LGA following a miscommunication with ATC. A wake turbulence encounter earlier in the descent was cited as contributing.
Synopsis
B737NG flight crew reported descending below cleared altitude on arrival into LGA following a miscommunication with ATC. A wake turbulence encounter earlier in the descent was cited as contributing.
Narrative
Descending into LGA on the Milton 4 arrival. We were given a clearance to cross MARRC intersection at FL180. I read that back and we started down on path to cross MARRC at FL180. As we got close to MARRC; we encountered very strong wake turbulence. The aircraft encountered moderate turbulence and an uncommanded rolling moment. The Autopilot disconnected and we lost guidance both vertically and laterally. We got that straightened out fairly quickly. In the midst of dealing with that; we were cleared to cross BEUTY at 11;000 ft. I read back BEUTY at 11;000 ft. as normal; and as normal there is no reply to that from ATC. I noticed at the time that the chart showed to 'expect' BEUTY at 13;000; but I didn't think much of that because many times we get altitudes other than the charted 'expect' altitudes. We descended normally to cross BEUTY at 11;000 ft. and we got a frequency change. We checked in at 11;000 ft. and got no answer as the frequency was very congested. I waited a while and tried again. The controller asked what altitude we are at; which immediately sets off alarm bells in my head and the First Officer (FO) as well. I replied that we are at 11;000 ft. and there was no reply. I waited a moment as the controller dealt with other aircraft and I asked him why he asked us. He responded that we were assigned 13;000 by the last controller. I replied that we heard 11;000; and at that time I had read back 11;000. He had nothing else to say to us on the matter. We continued on to LaGuardia as normal with no further discussion with ATC on the matter.On the post flight debrief; the FO concurred that while he had his hands full as flying pilot due to the wake turbulence encounter that he heard 11;000 as well; and he heard me read back the clearance to cross BEUTY at 11;000. Also; he agrees that ATC did not reply with any correction at that time.
Second reporter narrative
We were on the Milton 4 arrival and instructed to cross MARRC at 18;000. As we were descending; we encountered what we believe was wake turbulence at approximately 19;000 and around 18;500 the autopilot went into CWS P and CWS R. At this point I was hand flying the airplane and the Captain was in the process of turning the flight directors off then on and restoring LNAV/VNAV; when ATC gave us a crossing restriction at BILEY. I thought I heard the controller say cross BILEY at 11;000. The Captain read back cross BILEY at 11;000; then he set 11;000 in the mode control panel. With LNAV/VNAV and the Autopilot on; we continued our descent. We leveled at 11;000 and ATC asked what our assigned altitude was; the Captain responded 11;000; that we had been told to cross BILEY at 11;000. ATC had us turn left then back to the right. I mentioned to the Captain; I wonder why he asked our assigned altitude; with BILEY being charted as expect to cross at 13;000; I wanted to know if we got the 11;000 altitude wrong. He queried ATC and the Controller said he was expecting us to cross BILEY at 13;000; he didn't know why the previous Controller would have instructed us to cross BILEY at 11;000. We landed in LGA and taxied to the gate. With the turbulence or wake turbulence; the automation going into CWS P and CWS R we had a lot going on. I thought I heard the crossing restriction being 11;000 and that's what the Captain heard as well. It's possible that we misheard the instruction and we should have confirmed the altitude with ATC.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.